Field of the Invention
This invention relates to additives for the proton conducting polymer electrolyte used in membranes, catalyst layers, and the like in fuel cells. In particular, it relates to additives for improved durability and performance thereof.
Description of the Related Art
Sustained research and development effort continues on fuel cells because of the energy efficiency and environmental benefits they can potentially provide. Solid polymer electrolyte fuel cells show particular potential for use as power supplies in traction applications, e.g. automotive. However, various challenges remain in obtaining desired performance and cost targets before fuel cells are widely adopted for automotive applications in particular.
Solid polymer electrolyte fuel cells (also known as proton exchange membrane fuel cells) convert reactants, namely fuel (such as hydrogen) and oxidant (such as oxygen or air), to generate electric power. They generally employ a proton conducting polymer membrane electrolyte between two electrodes, namely a cathode and an anode. Appropriate catalyst compositions (typically supported platinum or platinum alloy compositions) are employed at each electrode to increase the reaction rate. A structure comprising a membrane electrolyte sandwiched between these two electrodes is known as a membrane electrode assembly (MEA). Porous gas diffusion layers (GDLs) are usually employed adjacent the two electrodes to assist in diffusing the reactant gases evenly to the electrodes. Further, an anode flow field plate and a cathode flow field plate, each comprising numerous fluid distribution channels for the reactants, are provided adjacent the anode and cathode GDLs respectively to distribute reactants to the respective electrodes and to remove by-products of the electrochemical reactions taking place within the fuel cell.
Water is the primary by-product in a cell operating on hydrogen and air reactants. Because the output voltage of a single cell is of order of 1V, a plurality of cells is usually stacked together in series for commercial applications. In such a stack, the anode flow field plate of one cell is thus adjacent to the cathode flow field plate of the adjacent cell. For assembly purposes, a set of anode flow field plates is often bonded to a corresponding set of cathode flow field plates prior to assembling the stack. A bonded pair of an anode and a cathode flow field plate is known as a bipolar plate assembly. Fuel cell stacks can be further connected in arrays of interconnected stacks in series and/or parallel for use in automotive applications and the like.
MEA durability is one of the most important issues for the development of fuel cell systems in either stationary or transportation applications. For automotive applications, an MEA is required to demonstrate durability of about 6,000 hours.
In such cells, the membrane electrolyte serves as a separator to prevent mixing of reactant gases and as an electrolyte for transporting protons from anode to cathode. Perfluorosulfonic acid (PFSA) ionomer, e.g., Nafion®, has been the material of choice to date and the technology standard for membranes. Nafion® consists of a perfluorinated backbone that bears pendent vinyl ether side chains, terminating with SO3H.
Failure of the membrane as an electrolyte will result in decreased performance due to increased ionic resistance, and failure of the membrane as a separator will result in fuel cell failure due to mixing of anode and cathode reactant gases. The chemical degradation of PFSA membrane during fuel cell operation is proposed to proceed via the attack of hydroxyl (—OH) or peroxyl (—OOH) radical species on weak groups (such as a carboxylic acid group) on the ionomer molecular chain. The free radicals may be generated by the decomposition of hydrogen peroxide with impurities (such as Fe2+) in a Fenton type reaction. In fuel cells, hydrogen peroxide can be formed either at Pt supported on carbon black in the catalyst layers or during the oxygen reduction reaction.
The hydroxyl radical attacks the polymer unstable end groups to cause chain zipping and/or could also attack an SO3− group under dry conditions to cause polymer chain scission. Both attacks degrade the membrane and eventually lead to membrane cracking, thinning or forming of pinholes. The membrane degradation rate is accelerated significantly with increasing of the operation temperature and with decreasing inlet gas relative humidity (RH).
Different additives to the membrane electrolyte have been studied for purposes of improving the performance and/or durability of the membrane. These additives include: 1) hygroscopic particles made of metal oxide, such as silica or zirconium dioxide, heteropoly acids, phosphonate silica, etc. to improve MEA performance under low RH conditions by increasing water retention (e.g. US20070154764); 2) Pt catalyst particles dispersed in the electrolyte membrane to improve membrane durability as well as membrane performance under low RH (e.g. US20070072036); 3) metal elements or compositions containing metal elements or metal alloys that act as a free radical scavenger or hydrogen peroxide decomposition catalyst (e.g. US2004043283); 4) phenol type antioxidants where the antioxidant can be a small molecule or a polymer (e.g. US2006046120); 5) organic crown compounds (e.g. US20060222921) or macrocyclic compounds containing metal or metalloids (e.g. WO2007144633); and 6) cation chelating agents to reduce formation of free radicals (e.g. U.S. Pat. No. 6,607,856).
Additives are also disclosed in WO2005060039 to address the problem in PEM fuel cell durability of premature failure of the ion-exchange membrane. The degradation of the ion-exchange membrane by reactive hydrogen peroxide species can be reduced or eliminated by the presence of an additive in the anode, cathode or ion-exchange membrane. The additive may be a radical scavenger, a membrane cross-linker, a hydrogen peroxide decomposition catalyst and/or a hydrogen peroxide stabilizer. The presence of the additive in the membrane electrode assembly (MEA) may however result in reduced performance of the PEM fuel cell. In particular, suggested additives include an organometallic Mn (II) or Mn (III) complex having an organic ligand selected from CyDTA, ENTMP, gluconate, N, N′-bis (salicylidene) propylenediamine, porphyrins, phthalocyanines, phenanthroline, hydrazine, pyrocatechol-3,5-disulphonic acid disodium salt, triethylenetetraamine, Schiff base macrocycles and EDDA.
In commonly owned published US patent application number US20110111321, certain ligand additives (e.g. 1,10-phenanthroline or 2,2′-bipyridine) were disclosed that meet many of these needs. The use of these ligand additives in the membrane and/or catalyst layers can improve durability but, depending on testing conditions, there may be a modest penalty in fuel cell performance (e.g. 3 times better stability might be obtained but with a 20 mV loss in voltage under load). Preferably, both durability and performance of fuel cells would be improved with appropriate additives.
In commonly owned published PCT application number WO2011/057769 (also US20120225361) which are incorporated herein by reference in their entirety, additives are disclosed which can be used to prepare polymer electrolyte for membrane electrode assemblies in polymer electrolyte fuel cells in order to improve both durability and performance. The additives are chemical complexes comprising certain metal and organic ligand components.
Accordingly, there remains a need for improved additive technology that provides additional resistance of MEAs, and particularly PFSA membranes of the MEAs, to degradation, resulting in improved MEA durability and performance in a fuel cell. This invention fulfills these needs and provides further related advantages.